The search that never ends

This search uses a closed interval. Which input makes the loop stop making progress?

while (left <= right) {
    int mid = left + (right - left) / 2;
    if (a[mid] == target) return mid;
    if (a[mid] < target) left = mid;
    else right = mid - 1;
}

Assume ordinary supporting code and valid inputs unless the question states otherwise.

Choose an answer before revealing the trace. Keys 1 to 3 work too.